Syrian President Bashar Assad formally agreed Thursday to surrender his chemical weapons arsenal, marking the first concrete step in a diplomatic process aimed at heading off a potential U.S. military strike against his regime.

Syrian President Bashar Assad formally agreed Thursday to surrender his chemical weapons arsenal, marking the first concrete step in a diplomatic process aimed at heading off a potential U.S. military strike against his regime.

U.N. Secretary-General Ban Ki-moon’s office welcomed Assad’s decision to sign onto an international convention against chemical weapons as U.S. and Russian officials gathered in Geneva to discuss the complex logistics of seizing and destroying Syria’s stockpiles in the midst of a ferocious civil war.

Despite the potentially historic nature of Syria’s submission – Syria was one of only seven U.N. members that had not signed the chemical weapons ban – the Obama administration received the news warily, noting the deep trust deficit that would have to be overcome before all parties can agree to a deal.

It’s clear the administration expects Moscow to follow up on Assad’s end of a deal. At the White House, spokesman Jay Carney said Russia President Vladimir Putin bore the primary responsibility for making sure Syria adheres to its obligations under the chemical weapons convention to declare its weapons stores and allow for their destruction.

“It is clear that President Putin has invested his credibility in transferring Assad’s chemical weapons to international control and ultimately destroying them,” Carney said. “This is significant. Russia is Assad’s patron and protector. And the world will note whether Russia can follow through on the commitments that it’s made."

But if the logistics behind the destruction of Assad’s chemical arsenal are complex – chemical weapons experts cautioned that destroying a stockpile in the midst of a war has never been tried before – the diplomatic track is also fraught with difficulties.

As U.S. Secretary of State John Kerry and Russian Foreign Minister Sergey Lavrov met in Geneva – a city synonymous with peace accords – their differences quickly became as clear as the waters of nearby Lake Geneva and the Alpine peaks reflected in it.

In a joint appearance, they disagreed on which side of the Syrian conflict has deployed chemical weapons and they clashed on how or if Assad should be punished should he fail to comply with international demands. And those were just two of the many contentious issues that must be resolved to assure Syria’s full chemical disarmament and to end threats of a U.S. missile strike.

Kerry rejected Assad’s notion, floated on Russian television, that Syria could win a 30-day grace period before inspections work begins by signing onto the convention. Kerry also dismissed Assad’s demand that the U.S. stop supporting Syrian rebels in order for any proposal to be credible.

Kerry said the Obama administration refused to take the threat of U.S. military action off the table.

“It has to be real. It has to be comprehensive. It has to be verifiable. It has to be credible,” Kerry said of any deal that would come from the Geneva talks. “It has to be timely and implemented in a timely fashion. And finally, there ought to be consequences if it doesn’t take place.”

For his part, Lavrov said Russia believed that heading off a U.S. military strike was the goal of the talks. “We proceed from the fact that the solution of this problem will make unnecessary any strike on the Syrian Arab Republic,” he said.

The atmosphere of the talks couldn’t seem bleaker. Just a week ago, Putin had called Kerry a liar. On Thursday, he wrote a column published in The New York Times in which he warned that a U.S. strike would kill innocent Syrians, widen the conflict, undermine diplomacy on Middle East peace and Iran’s nuclear program, and “unleash a new wave of terrorism.”

For their part, U.S. officials have called Russia “obstructionist” and said that Moscow is siding with child killers. Administration officials also have begun repeating a Reagan-era slogan when it comes to Russia: “Trust, but verify.”

Meanwhile, on Russian television, Assad – accused of war crimes by U.S. officials – flatly said that he doesn’t trust the United States.

Such public discord, coupled with the complex maneuvering it would take to dismantle an arsenal said to contain chemical agents sarin, mustard gas and VX in the midst of a civil war, doesn’t portend a breakthrough in Geneva.

Still, Thursday was the first concrete step Syria has taken to acknowledge that it possesses chemical weapons, and while no side expressed a willingness to trust one another, all sides had a common goal – avoiding a strike that polls show is unpopular among Americans.

Assad, too, is determined to avoid a strike. On Thursday, his envoy to the U.N. presented the Syrian leader’s signature on the Convention on the Prohibition of the Development, Production, Stockpiling and Use of Chemical Weapons and on their Destruction.

“The time has come for Syrians to join the OPCW,” said Bashar Jaafari, referring to the Organization for the Prohibition of Chemical Weapons, the international body that monitors the world’s chemical weapons ban.

Allam reported from Washington, Schofield from Geneva. Anita Kumar in Washington and McClatchy special correspondent Aaron Morrison at the United Nations in New York contributed to this report.